1. Field of the Invention
The present invention relates to a fiber spreading system for spreading a fiber bundle consisting of a plurality of filaments.
2. Description of the Related Art
As conventional fiber spreading apparatuses, there are known one using an electrostatic fiber spreading method, one using a fiber pressing and spreading method, one using a fiber jet-spreading method, and one using an ultrasonic fiber spreading method. Among them, the fiber spreading apparatus using the ultrasonic fiber spreading method includes an ultrasonic generator in a predetermined liquid tank and a fiber bundle streaming and feeding section that streams and feeds a to-be-spread fiber bundle to this liquid tank, and that spreads the fiber bundle using a ultrasonic wave, as disclosed in Japanese Unexamined Patent Publication Nos. 70420 (1992) and 145556 (1995).
Nowadays, a fiber bundle consisting of an assembly of filaments, which are carbon fibers, is used to obtain a composite material semi-finished item such as a prepreg. A fiber spreading degree required for this fiber bundle is rapidly increased. For example, an untwisted carbon fiber bundle consisting of 12,000 filaments of 7 μm and having an original width of about 6 mm and an original thickness of about 0.13 to 0.16 mm is required to be spread to have a width of about 25 mm and a thickness of about 0.02 mm in a final fiber-spread state.
Under these circumstances, the inventor of the present invention proposed a fiber spreading system, identified by Japanese Patent No. 3382607. This fiber spreading system includes a preliminary fiber spreading apparatus and a regular fiber spreading apparatus. The preliminary fiber spreading apparatus includes a fiber bundle streaming and feeding section provided in a liquid for streaming and feeding a fiber bundle to a plurality of rollers to follow a bent path while a tension is applied to the fiber bundle and the fiber bundle is brought into contact with surfaces of the respective rollers, propagates an ultrasonic wave into the liquid, and spreads the fiber bundle in the fiber bundle streaming and feeding section. The regular fiber spreading apparatus further spreads the spread fiber bundle spread by the preliminary fiber spreading apparatus.
If fine filaments of about 7 μm are to be arranged to have the width of about 25 mm and the thickness of about 0.02 mm in the final fiber-spread state, then about 3600 filaments are arranged in a width direction but only about three to four filaments are arranged in a thickness direction. If the number of filaments in the thickness direction is far smaller than that in the width direction, the fiber bundle is more liable to be split apart.
For example, in the preliminary fiber spreading apparatus of the above-stated fiber spreading system, a fiber spreading action in the fiber bundle streaming and feeding section is increased with passage of time. If fiber spreading time is too long, the fiber bundle may possibly be already split apart at the time the fiber bundle is discharged from the fiber bundle streaming and feeding section. Due to this, the fiber spreading system is required to execute two-stage steps by the preliminary fiber spreading apparatus and the regular fiber spreading apparatus, respectively, and to excessively suppress the fiber spreading time of the preliminary fiber spreading so that the preliminary fiber spreading apparatus performs only preliminary fiber spreading.
In this fiber spreading system, the fiber is spread while being immersed in a liquid. If the fiber bundle is discharged outside of the liquid after the fiber spreading, then the filaments overlap one another by a surface tension of the liquid adhering to the fiber bundle, and the fiber bundle is split apart. To avoid such a disadvantage, the above-stated fiber spreading system includes a squeezing roller mechanism. The squeezing roller mechanism consists of a metal roller a part of which is immersed in the liquid and a rubber roller abutting this metal roller from above, and removes the liquid adhering to a spread-fiber sheet by causing the spread-fiber sheet to pass between the both rollers (see Japanese Patent No. 3382607, paragraph [0046]). However, if liquid removal efficiency of this squeezing roller mechanism is low, the fiber bundle is often split apart by the surface tension of the liquid even after the liquid is squeezed.